Hello,
The max_age timer is how often a switch actually saves it's BPDU
information. It is part of the your overall STP convergence when there is
an INDIRECT link failure. Imagine the following simple network:
SWA
/ \
SWB----SWC
Assume SWA is the root bridge. Therefore, all it's ports are forwarding.
Assume SWB has the designated port on the SWB/SWC segment so it's port
facing SWC is forwarding. The SWB/SWA and SWC/SWA links are root ports. The
port facing SWB on SWC is blocking. Now imagine the SWA/SWB link goes down.
Now SWB does not get BPDUs from the root because the link failed...it also
isn't receiving any BPDUs from the path SWA --> SWC --> SWB because the link
facing SWB on SWC is in the blocking state currently. Therefore, SWB
basically thinks he is the root and starts sending inferior BPDUs towards
SWC. THIS is where max_age comes in. SWC will IGNORE these BPDUs until
max_age expires (20 seconds by default). After max_age expires,
spanning-tree will converge -- SWC will become the designated port on the
SWB-SWC segment.
To answer your question -- When you have an INDIRECT link failure you must
wait max_age + 2x forward-delay to converge. So, if you are dealing with an
indirect link-failure and you want to tweak how fast STP takes to converge,
you can play with forward-delay , max_age or both. Lets say you wanted STP
to converge in this example in 14 seconds instead of 50 seconds. You could
make max_age = 4 seconds and forward-delay = 5 seconds so max_age + 2x
forward-delay would be 14 seconds.
2009/12/8 Iwan Hoogendoorn <[email protected]>
> César,
>
> 1 question at the time :-)
> Can you be more specific with your question?
>
> --
> Regards,
>
> Iwan Hoogendoorn
> CCIE #13084 (R&S / Security / SP)
> Sr. Support Engineer – IPexpert, Inc.
> URL: http://www.IPexpert.com
>
>
>
> 2009/12/8 César Martínez Segura <[email protected]>:
> > Hi guys,
> >
> > Could anybody explain me exactly the meaning of this parameter? For my
> > readings, max-age is the time that a switch store a BPDU. Knowing that,
> > seeing a question is IPExpert asking for how a switch can detect the use
> of
> > a link in 10s, the answer is configuring in all the switches --> span
> vlan
> > 1-4094 max-age 10. Could anybody explain me why? And, apart from that,
> why I
> > don't have to take care about the forwarding delay?
> >
> > Thanks,
> > Cesar.
> >
> > _______________________________________________
> > For more information regarding industry leading CCIE Lab training, please
> > visit www.ipexpert.com
> >
> >
> _______________________________________________
> For more information regarding industry leading CCIE Lab training, please
> visit www.ipexpert.com
>
--
Regards,
Joe Astorino CCIE #24347 (R&S)
Sr. Technical Instructor - IPexpert
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